Elucidating the mechanisms by which hematopoietic stem cell renewal and lineage differentiation are controlled is crucial to improving hematopoietic recovery after stress and hematopoietic cell transplantation. Recently, we discovered that the biochemically distinct and unique DEK protein is a factor that enhances ex-vivo expansion of hematopoietic stem cells (HSCs) and HSC engraftment, while regulating hematopoietic progenitor cell (HPC) proliferation and cycling though its interaction with the CXCR2 chemokine receptor. While DEK has been implicated in multiple intranuclear activities, we have recently found that this non-histone protein with no enzymatic activity modulates global heterochromatin integrity and hence gene expression. Interestingly, in addition to its intranuclear functions, we have found that DEK can be secreted by monocytic cells and released by apoptotic T cells, and act as a chemotactic factor for neutrophils, as well as for HSCs and HPCs, through a mechanism involving CXCR2, which also mediates at least some of DEK's effects on the cycling of HPCs. Remarkably, we have now also discovered that the intranuclear and extracellular functions of DEK can be unified, as extracellular DEK can be taken up by several different types of hematopoietic and other cells in a bioactive form that modulates chromatin structure and DNA repair. Thus, DEK is a nuclear protein that regulates hematopoiesis and participates in a highly unusual loop involving secretion, receptor engagement, uptake, and subsequent modulation of heterochromatin biology, gene expression and HSC/HPC functions. Strikingly, this loop is active in vivo, as subcutaneous administration of recombinant DEK to mice leads to significant increases in phenotyped HSCs and lower numbers and decreased cycling of HPCs. Therefore, we propose to use a multidisciplinary approach to test the hypothesis that this intranuclear/extracellular loop involving DEK plays a significant role in hematopoietic growth and differentiation and stem engraftment by affecting chromatin balance and CXCR2-mediated signaling. Using immature subsets of mouse and human hematopoietic cells, mouse models of homeostasis and stressed hematopoiesis, cell signaling studies, epigenetic analysis, and genome-wide studies of chromatin structure and gene expression, we will further delineate the mechanisms by which DEK modulates the fate decisions between HSC proliferation, survival, self-renewal and differentiation and promotes HSC engraftment in vivo. The proposed experiments will elucidate how DEK works through CXCR2 signaling and the highly unusual extracellular/intracellular chromatin loop to affect HSC fate decisions and hematopoiesis. This will be done in context of Dipeptidylpeptidase (DPP) 4 modification of DEK, and the influence of extra physiologic oxygen shock/stress on DEK functions in vivo and in vitro. These studies have the potential to generate new approaches to the treatment of hematopoietic disorders and other diseases in which hematopoietic cell transplantation is required.